1. Field on the Invention
The present invention relates to a method of increasing the leaching yield of gold in the cyanide treatment of refractory ores or concentrates of such ores while delivering air in an alkaline environment at atmospheric pressure, said ores or concentrates containing gold that is bound as tellurides.
2. Description of the Related Art
Gold is found partly in solid form, i.e. as pure metal, and partly in combination with other elements, for instance in the form of alloys and compounds. Finely divided ores that have low gold concentrations are conventionally treated hydrometallurgically, primarily by the so-called cyanide process in which the gold is dissolved in a sodium cyanide solution to form a cyanide complex. Gold is won from the resultant leaching solution, by precipitation with suitable less precious metals, for example finely divided zinc, or by adsorption on active carbon.
Refractory ores and concentrates are materials that cannot be processed readily to recover their valuable metal content, for instance materials in which gold and other valuable metals are bound as certain tellurides. Conventional cyanide leaching of ores and concentrates containing gold tellurides often result in slow kinetics and incomplete dissolution of the gold content. A large number of gold tellurides exist, of which many exhibit strong refractory properties. In the event that efforts are made to win the valuable metal content of such refractory materials with a cyanidation process, complicated and often expensive pre-treatment of the material is required, for instance chemical pre-treatment or roasting.
In addition to the extra treatment steps required, other drawbacks with the roasting of such materials reside in the emission of SO2 and As2O3 in the roaster gas, sintering of the roasted product, coupled with the fact that the oxidation and reduction process is difficult to control. However, such pre-treatment is applied commercially at present in a plant on Fiji (Emperor Gold Mine) in combination with hypochlorite treatment.
In addition to hypochlorite treatment, other proposed chemical pre-treatment methods include leaching in a strong acid, for instance in hydrochloric acid, in order to remove tellur in an additional step prior to the cyanide process.
A large number of pre-treatment methods for refractory gold ores have been described in the literature since the end of the nineteenth century. These methods have been combined in a lecture at Randol 3 (Proceedings Gold and Silver Recovery Forum 87, pp. 1885-97). The processes proposed include oxidation with Ca(OCl)2, comminution, cyaniding with large quantities of lime, and cyaniding at elevated pressures in an autoclave. The article also makes apparent that these materials cannot be treated pyrometallurgically because of the excessively high gold losses that occur as a result of vaporisation or absorption in the brick lining of the surrounding vessel, among other things.
In conjunction with creating and developing methods for processing ores from our deposit in xc3x96stra xc3x85kulla, Sweden, where large concentrations of gold tellurides exist, it was surprisingly found possible to elevate the leaching yield of this material with cyanide treatment in conventional leaching circuits in the absence of any additional steps, e.g. by leaching at normal atmospheric pressure in the absence of any pre-treatment, to obtain a practically perfect leaching yield.
The method according to the invention is characterised in this regard by the method steps set forth in the accompanying Claims.
Thus, in accordance with the invention, the cyanide treatment is carried out at a temperature above room temperature up to a temperature immediately beneath the boiling point of the leaching liquid. The treatment is suitably carried out at a temperature of 60-80xc2x0 C. A temperature of about 75xc2x0 C. has been found particularly effective.
The treatment is carried out while delivering air in an alkaline environment, preferably at a pH between 10 and 12.